J . Am. Chem. SOC.1986, 108, 1579-1585 Preparation of Ions. Cyanocarbenium ions and propargyl cations were prepared from the progenitor cyanohydrins and propargyl alcohols as described earlier.'2*'4 The I3C NMR spectroscopic studies were carried out at 50 MHz on a Varian XL-200 superconducting NMR spectrometer equipped with a variable-temperature broad-band probe. The 13C-13Ccoupling constants in the 13C-enrichedions and the progenitors were directly measured from their proton-noise-decoupled I3C NMR spectra. The coupling constants in the other neutral model compounds reported in Table I1 were mea-
1579
sured from the "C satellite spectra by using the INADEQUATE6 pulse ~ T-90' (x)- A-90' (@)=Acq. ($), sequence: 90° (x)- ~ - 1 8 0(&y)where r N ( 2 n + 1)/4Jcc, A is a very short delay (-10 ks) needed to reset the radiofrequency phase during which time double-quantum coherance evolves and @ and $ are the phase of the last 90" "read" pulse and the receiver, respectively.
Acknowledgment. Support of our work by the National Institutes of Health is gratefully acknowledged.
Essentially Solvent-Independent Rates of Solvolysis of the 1-Adamantyldimethylsulfonium Ion. Implications Regarding Nucleophilic Assistance in Solvolyses of tert-Butyl Derivatives and the NKLSolvent Nucleophilicity Scale' Dennis N. Kevill* and Steven W. Anderson Contribution from the Department of Chemistry, Northern Illinois University, DeKalb, Illinois 601 15. Received September 3, 1985
Abstract: Rearside nucleophilic solvation of the developing carbocation is severely hindered during the solvolysis of the 1-adamantyldimethylsulfoniumion (1 -AdSMe2+). For variation in the composition of binary solvent systems, the ion always shows a variation of the specific rate in the opposite direction to that observed for the tert-butyldimethylsulfonium ion (t-BuSMe2+!; these observations are rationalized in terms of a dominant nucleophilic solvation at the transition state for r-BuSMe2+solvolysis and a dominant nucleophilic stabilization of the reactant for l-AdSMe2+ solvolysis. These systems are compared to the corresponding alkyl halides. Corrections applied in setting up the NKLscale of solvent nucleophilicities would be better based on l-AdSMe2+ solvolyses. However, the insensitivity of 1-AdSMe2+solvolyses to solvent variation (specific rates in 41 solvents, at 70.4 O C , vary by less than a factor of seven) allows log (k/ko)Et30+values to be taken as a good measure of solvent nucleophilicity. For solvolyses of l-AdSMe2+ in aqueous ethanol (260% ethanol), the S value (faitoring reaction with water) is 1.35 at both 70.6 and 100.1 O C .
Considerable interest centers around the extent to which specific rates of solvolysis of tert-butyl chloride are influenced by the nucleophilicity of the solvent. Evidence for2-I0 and against"-I6 detectable interaction of the solvent has been presented. The original Grunwald-Winstein equation17 (eq 1) defines a scale of log ( k / k , ) = m Y
(1)
( I ) (a) Presented at the Seventh IUPAC Conference on Physical Organic Chemistry, Auckland, New Zealand, Aug 20-24, 1984, Abstract C2. (b) Abstracted, in part, from the Ph.D. Dissertation of S.W.A., Northern Illinois University, Aug 1985. (2) Ross, S. D.; Labes, M. M. J . A m . Chem. SOC.1957, 79, 4155. (3) Frisone, G. J.; Thornton, E. R. J . A m . Chem. SOC.1968, 90, 1211. (4) Mohanty, R. K.; Robertson, R. E. Can. J . Chem. 1977, 55, 1319. (5) Harris, J. M.; Mount, D. L.; Smith, M. R.; Neal, W. C., Jr.; Dukes, M. D.; Raber, D. J. J . A m . Chem. Sor. 1978, 100, 8147. (6) Parker, A. J.; Mayer, U.; Schmid, R.; Gutmann, V. J . Org. Chem. 1978. 43. 1843.
(7) Bentley, T. W.; Bowen, C. T.; Parker, W.: Watt, C. I. F. J . Am. Chem.
SOC.1979, 101, 2486.
(8) Kevill, D. N.; Kamil, W. A,; Anderson, S. W. Tetrahedron Lett. 1982, 23, 4635. (9) Bentley, T. W . ; Carter, G. E. J . Am. Chem. SOC.1982, 104, 5741. (IO) Swain, C. G.;Swain, M. S.; Powell, A. L.; Alunni, S. J . A m . Chem. SOC.1983, 105, 502. ( I 1) Raber, D. J.; Bingham, R. C.; Harris, J . M.;Fry, J. L.; Schleyer, P. v. R. J . Am. Chem. SOC.1970, 92, 5977. (12) Scott, J. W.; Robertson, R. E. Can. J . Chem. 1972, 50, 167. (13) Koppel, I. A,; Palm, V. A. In 'Advances in Linear Free Energy Relationships"; Chapman, N. B., Shorter, J., Eds.: Plenum Press: London, 1972; p 203. (14) Fawcett, W. F.; Krygowski, T. M. Ausr. J . Chem. 1975, 28, 2115. ( 1 5 ) Abraham, M. H.; Taft, R. W.; Kamlet, M . J . J . Org. Chem. 1981, 46, 3053. (16) Dvorko, G. F.; Ponomareva, E. A,; Kulik, N. I. Russ. Chem. Rec. 1984, 53, 948. (17) Grunwald, E.; Winstein, S. J . Am. Chem. SOC.1948, 70, 846.
0002-7863/86/1508-1579$0l.50/0 , I
,
solvent ionizing power (Y) in terms of the specific solvolysis rates of tert-butyl chloride in a given solvent ( k ) and in 80% ethanol ( k o ) ,with m assigned a value of unity. Accordingly, the detailed mechanism has important implications both as regards the significance of the Y scale and as regards the types of substrates for which it can logically be used in correlation analysis. In a discussion of solvation effects upon SN1 and E l reactions, Ingold statedI8 "in the unimolecular substitution of an alkyl halide, both of the ions, which are formed in the initial heterolysis, have to be solvated". While the nucleophilic solvation of a developing bridgehead carbocation will necessarily be relatively weak, appreciable rearside solvation is possible for a developing tert-butyl c a r b o c a t i ~ n . ' In ~ ~accord ~ ~ with the description of Ingold, we have considered8 the solvolysis of tert-butyl chloride to involve carbenium ion formation, with the need to consider solvation of both of the incipient ions. Bentley and c o - w o r k e r have ~ ~ ~ ~viewed the mechanism in a slightly different way and have formulated it as an example of a n SN2 (intermediate) mechanism.*' Other workersi3-l6 have, however, proposed that changes in the specific solvolysis rate, accompanying changes in the solvent, can be rationalized without the need to consider the nucleophilicity of the solvents. If our view of the solvation is accepted, the original Y scale17 is valid as a scale of solvent ionizing power. However, it should be used within eq 1 only for the correlation of solvolyses within which a similar (relatively small) contribution from nu(18) Ingold, C. K. "Structure and Mechanism in Organic Chemistry"; Cornell University Press: Ithaca, NY, 1953, p 357. (19) Okamoto, K. Pure Appl. Chem. 1984, 56, 1797. (20) DeTar, D. F.; McMullen, D. F.; Luthra, N. P. J . Am. Chem. SOC. 1978, 100, 2484. (21) Bentley, T. W.; Schleyer, P. v. R. J . Am. Chem. SOC.1976, 98, 7658.
0 1986 American Chemical Society
Kevill and Anderson
1580 J . Am. Chem. Soc., Vol. 108, No. 7, 1986 cleophilic solvation accompanies a dominant contribution resulting from electrophilic solvation and solvent dipolarity/polarizability effects. For example, a linear correlation of the logarithms of the specific rates of solvolysis of benzoyl chloride against Y values in solvents of high Y and/or low N values was considered22to reflect a nucleophilic contribution similar to that ~ l a i m e dfor ~,~ tert-butyl chloride solvolysis. For SN2 solvolyses with an appreciable dependence upon solvent nucleophilicity, a two-term equation (the extended GrunwaldWinstein equation) has been proposed (eq 2).23 The additional log ( k / k , ) = IN mY (2)
+
IN term involves a scale of solvent n u ~ l e o p h i l i c i t y(N) ~ ~ ~and ~ ~ the sensitivity ( I ) of the substrate solvolysis to this property. Ideally, all nucleophilic contributions are governed by the IN term, and any inclusion of nucleophilic properties within the Y scale is undesirable. Alternative Y scales have been developed based upon the solvolyses of adamantyl derivatives, where nucleophilic contributions are minimized.26 When a Ycl scale, based on 1adamantyl chloride solvolysis, is used within eq 2, a 1 value of ca. 0.3 (based upon NOTsvalues24) can be calculated for tert-butyl ~ h l o r i d e .The ~ validity of this analysis has been questioned. It has been s ~ g g e s t e d ~that ~ J the ~ moderate differences in response to the solvent variation observed for the specific rates of solvolysis of 1-adamantyl chloride and tert-butyl chloride could result not from a higher response to changes in the solvent nucleophilicity for tert-butyl chloride but from a higher response to changes in the solvent electrophilicity for 1-adamantyl chloride. Although the explanation in terms of nucleophilicity conforms nicely to a comparison of bridgehead and acyclic substrates, the strong covariences8qz7often observed between N and Y values, for a given mixed solvent system, make the choice between the two explanations a nontrivial one. However, the value interrelating the N and Y scales varies considerably for different binary systems, and the covarience problem can be overcome by a careful choice of standard solvent^.^^^^.^^ Support for the absence of a nucleophilic component comes from an alternative linear free-energy relationship (LFER) treatment of tert-butyl chloride solvolysis in terms of solvatochromatic properties (n*,a,and fl).29 It was foundI5 that the specific rates could be correlated against A* and a, without the need to include a term relating to solvent basicity (@values). Similarly, a treatmentI6 in terms of the Koppell-Palm equation,13 with inc l u ~ i o nof~ the ~ Hildebrand parameter, did not require a term relating to basicity or nucleophilicity. A problem with multiparameter equations is that inclusion of a minor contributor will not meaningfully improve the correlation if its contribution is significantly less than the sum of the deviations associated with the other parameters. For tert-butyl chloride, it is generally agreed that solvent-induced rate variations are governed primarily by electrophilicity and dipolarity/polarizability changes (for aqueous ethanol, a contribution of 10% being governed by nucleophilicity changes was estimated*). While multiparameter LFER treatments are needed for estimating the relative importance of electrophilicity and dipolarity/polarizability effects during tert-butyl chloride solvolysis, we believe that for solvolysis of tert-butyl derivatives, the best approach toward estimating the importance of nucleophilicity is not to try to improve the LFER treatment (such as by incorporation of additional terms) ~~
~~
(22) Bentley, T.W.; Carter, G.E.; Harris, H. C. J . Chem. Soc., Chem Commun. 1984, 387. (23) Winstein, S.; Grunwald, E.; Jones, H. W. J . A m . Chem. Sac. 1951, 73, 2700. (24) Schadt, F. L.; Bentley, T. W.; Schleyer, P v. R. J . Am. Chem. Soc 1976, 98, 7667. (25) Kevill, D. N.;,Lin, G. M. L. J . A m . Chem. Soc. 1979, 101, 3916. (26) For a recent discussion, with a tabulation of leading references, see Bentley, T. W.; Carter, G. E.; Roberts, K. J . Org. Chem. 1984, 49, 5183. (27) Kaspi, J.; Rappoport, Z . Tetrahedron Lett. 1977, 2035. (28) Kevill, D. N.; Rissmann, T. J. J . Chem. SOC.,Perkin Trans. 2 1984, 717
(29) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Chem. 1981, 14, 485. (30) Abraham, M. H. Prog. Phys. Org. Chem. 1974, I I , 1.
but to change the leaving group in a way which eliminates or minimizes those other contributions which are dominant in the solvolysis of tert-butyl chloride. A convenient type of substrate for such a study is with an initially positively charged leaving group, which leaves as a neutral molecule (eq 3). We have shown R-X+
-+
R + + :X
(3)
previously* that specific solvolysis rates for the tert-butyldimethylsulfonium ion (t-BuSMe2+) correlate with NKLsolvent nucleophilicity values,2s with an 1 value of ca. 0.35. For studies in aqueous ethanol, a brief comparison was made with specific solvolysis rates of the 1-adamantyldimethylsulfonium ion (1 AdSMe2+). In the present paper, we present full details of our study of the solvolysis of the l-AdSMe2+ ion (eq 4). It will be shown that the hindrance to rear-side solvation, due to the cage structure, leads to the specific solvolysis rates at a given temperature being almost independent of the composition of the solvent. For Sh2-reacting
SOH2+ (41
R-X+ substrates, the corrections which must be applied to the logarithmic rate ratio term in the establishment of an N scale are discussed. Product ratios in aqueous ethanol are determined and compared to those for neutral adamantyl substrate^.^'-^^ The ratios of t-BuSMe2+ to l-AdSMe2+ specific solvolysis rates in (HFIP) solvents are aqueous 1,1,1,3,3,3-hexafluoro-2-propanol obtained and compared with the low ratios previously reported' for the corresponding solvolyses of the chlorides.
Results Solvolyses in Dry Alcohols, Water, Acetic Acid, and 80% Ethanol. Rates of solvolysis were determined a t four or five temperatures in the 50-90 O C range. Runs in tert-butyl alcohol were carried out in the presence of a 5% excess of pyridine.37 In acetic acid, the reaction appeared to come to an equilibrium (paralleling observation^^^ for the acetolysis of the tert-butylethylmethylsulfonium ion) and theoretical infinity titers were used in the calculations; for example, for a 0.0063 M solution of 1AdSMe2+OTf in acetic acid a t 90.1 "C, a mean of experimental infinity titers from six independent kinetic runs corresponded to an acid development which was 68.6 0.8% of the theoretical. Addition of a 15% excess of sodium acetate prevented the establishment of an equilibrium and, at 70.4 O C , a specific acetolysis rate of 3.19 (i0.25)X 10" s-l was observed, a value 19% higher than that estimated (from data a t other temperatures) in the absence of sodium acetate. Constant integrated values for the first-order rate coefficients were obtained throughout each run. All the values for, at least, duplicate runs were averaged, and these averages are reported in Table I, together with values for Y+ (calculated by using eq 1 with m = 1) for l-AdSM%+ solvolysis. These U, values establish a scale analogous to the solvent ionizing power scale for neutral substrates. The activation parameters (at 70.4 "C), obtained from a treatment of the data of Table I in terms of the Eyring equation, are reported in Table 11.
*
(31) Harris, J. M.; Becker, A,; Fagan, J. F.; Walden, F. A. J . Am. Chem. SOC.1974, 96, 4484.
(32) Karton, Y . ; Pross, A. J . Chem. Soc., Perkin Trans. 2 1978, 595. (33) Luton, P. R.; Whiting, M. C. J . Chem. SOC.,Perkin Trans. 2 1979, 646. (34) Kevill, D. N.;Bahari, M. S.; Anderson, S. W. J . A m . Chem. SOC. 1984, 106, 2895. (35) McManus, S. P.; Zutaut, S. E. Tetrahedron Lett. 1984, 25, 2859. (36) Kevill, D. N.; Anderson, S. W. J . Org. Chem. 1985, 50, 3330. (37) Kevill. D. N.; Kolwyck, K. C . ;Weitl, F. L. J . Am. Chem. SOC.1970, 92, 7300. (38) Scartazzini, R.; Mislow, K. Tetrahedron Lett. 1967, 2719.
J . Am. Chem. SOC.,Vol. 108, No. 7 , 1986 1581
Solvolysis of the I -Adamantyldimethylsulfonium Ion
Table I. First-Order Rate Coefficients for the Solvolysis of 1-AdamantyldimethylsulfoniumTrifluoromethanesulfonateo*band y+ Valuesc
106k. s-' temp, "C 49.9 50.6 60.3 70.4 80.1 90.4 Y-+
80% EtOH 0.0770 0.461 2.27 10.6 45.8 0.000
MeOH 0.0996 0.520 2.47 12.0 0.037
EtOH 0.0765d 0.0802 0.423 2.15 10.3 -0.024
i-PrOH 0.0583d 0.063Sd 0.346 1.68 7.41 34.0 -0.131
r-BuOH 0.0331
AcOH
0.226 1.09 4.94 18.5 -0.3 19
0.479 2.45' 9.69 39.8 0.072
H2O 0.174 0.91 1 4.10 16.4 0.257
"Concentration of 0.0060-0.0065 M. bStandard deviations for k were within 5% of the reported values. clog (k/ko)I.AdSMe2+ at 70.4 "C, where k , refers to the specific rate in 80% ethanol. dSinale determination. 'At 69.8 "C; use of the Arrhenius equation leads to a value of 2.68 X s-' at 70.4 "C. Table 11. Enthalpies ( A H ' ) and Entropies (AS') of Activation for Solvolysis of 1-Adamantyldimethylsulfonium Trifluoromethanesulfonate"
Table 111. First-Order Rate Coefficients for the Solvolysis of the 1 -Adamantyldimethylsulfonium in Pure and Binary Solvents',d at 70.4 "C and P Values'
AH*,*,6. kcal/mol AS*2dl&, eu 36.2 f 0.2 20.9 f 0.6 35.2 f 0.8 17.9 f 2.6 36.1 f 0.6 20.3 f 1.8 36.0 f 0.4 19.6 f 1.1 35.8 f 0.6 18.2 f 2.0 34.4 f 0.9 15.7 f 2.9 34.2 f 0.4 16.2 f 1.3 "Calculated by using the data of Table I, with associated standard errors. *On a volume-volume basis at 25.0 "C.
106k, s-I P solvent 1o6k. s-' y+ 2.57 0.054 95% acetone 3.16h 0.144 2.87 0.102 90% acetone 3.1 1 0.137 3.56 0.195 80% acetone 0.124 3.02 2.79 0.090 60% acetone 0.106 2.90 2.85 0.099 40% acetone 3.08 0.133 0.177 3.01 0.123 20% acetone 3.41 -0.235 3.21 0.150 95% dioxane 1.32 I .69 -0. I28 6.49 0.456 90% dioxane 1.76 -0.1 1 1 5.58 0.391 80% dioxane 2.07 5.27 -0.040 0.366 70% dioxane 4.97 2.29 0.004 0.340 60% dioxane 2.30 4.43 0.006 0.290 40% dioxane 7.09' 2.71 0.077 0.495 20% dioxane 4.88 0.487 80% T-20% E' 6.97 0.332 3.47 0.356 60% T-40% E 5.15 0.184 0.332 40% T-60% E 0.121 4.87 3.00 0.035 20% T-80% E 2.46g 2.59 0.057 Concentration of 0.0060-0.0065 M. 'Standard deviations for k were within 6% of the reported values. 'Other component water, except for TFE-EtOH (T-E) mixtures. a volume-volume basis at 25.0 "C, except for TFE-H,O and HFIP-H,O mixtures which are on a weight-weight basis. eFor definition, see footnote c of Table I . 'Value of 0.304 X s-' s 49.7 "C. gObtained by extrapolation to zero [ H C 0 2 N a ] of three duplicate determinations with added sodium formate; with [ H C 0 2 N a ] of 0.0040, 0.0086, and 0.0171 M, 106k values were 2.49, 2.59, and 2.66 s-'. With initial [Me,S] of 0.050 and 0.10 M , 106k values were 3.09 and 2.94 s-!
solvent 80% ethanolb methanol ethanol 2-propanol rerr-butyl alcohol acetic acid water
Solvolyses in Solvents of Varying Ionizing Power. A study was made at 70.4 OC in formic acid, 2,2,2-trifluoroethanoI (TFE)ethanol mixtures and in mixtures of water with ethanol, methanol, acetone, TFE, dioxane, and HFIP. For solvolysis in 100 and 97% TFE and in 97 and 90% HFIP, a 5% excess of pyridine was added, to prevent the establishment of an equilibrium. Those values not previously reported in Table I are reported in Table 111. All runs were performed, a t least, in duplicate. For solvolysis in 95% acetone, the effect upon the specific solvolysis rate of initially added dimethyl sulfide, in concentrations of up to 0.1 M, was found to be negligible (Table 111). In formic acid, a rather early equilibrium could be avoided by the addition of sodium formate. Specific rates in the presence of three concentrations of sodium formate were linearly extrapolated to zero concentration (Table 111). Product Studies. The products from the solvolysis of 1AdSMe,+OTf in aqueous ethanol mixtures containing from 96 to 20% ethanol were studied by GLPC a t 70.6 and 100.1 OC. A 10% excess of pyridine was added in order to prevent acid-catalyzed equilibration of the alcohol and ethyl ether products, which had been observed for the corresponding solvolyses of other adamantyl derivative^.^^,^^ No evidence was obtained for any 1adamantyl methyl sulfide formation, which would have been expelled during substitution reaction a t a methyl group. The percentages of 1-adamantan01 formation are reported in Table IV. The small percentages of 1-adamantan01 found after solvolysis in 100%ethanol were ascribed as being due to contamination of the substrate. In the calculation of the selectivity values (S), reported in Table IV, the percentage of I-adamantanol found after solvolysis in 100% ethanol was subtracted from each of the values reported for the solvolysis in aqueous ethanol before insertion into eq 5 . [adamantanol] [ethanol] S = [adamantyl ethyl ether] [water] (5) Effect upon the Ethanolysis of Added Tetraethylammonium Chloride. Addition of 0.16 M tetraethylammonium chloride to the ethanolysis of 0.0062 M l-AdSMe2+OTf, at 70.4 "C, severely perturbed both the kinetics and the product formation. The specific rates of acid formation, based upon theoretical infinity titers, fell in value during the run. The development of acid reached a maximum value of ca. 45% of the theoretical after 8 days, and it then fell slowly in value to about 30% of the theoretical
solvent
60% EtOH 40% EtOH 20% EtOH 80% MeOH 60% MeOH 40% MeOH 20% MeOH 100% TFE 97% T F E 90% T F E 70% T F E 50% T F E 97% H F I P 90% H F I P 70% H F I P 50% H F I P 100% H C 0 , H
after 78 days. A GLPC analysis of the products indicated the absence of 1-adamantyl chloride and the presence of a 60% yield of 1-adamantyl methyl sulfide. Solvolysis of terf-Butyldimethylsulfonium Trifluoromethanesulfonate in HFIP-H20Mixtures. Our previous ~ t u d i e s of * ~the ~~ solvolysis of this salt have been extended to 9 7 4 0 % HFIP at 49.7 and 70.1 "C. The specific solvolysis rates are reported in Table V, together with ratios for the reactivity of this substrate relative to the reactivity of l-AdSMe,+OTf in identical solvolyses (using specific rates from Table 111).
Discussion Although the I-AdSMe2+ ion does not appear to have been previously reported, there are two reports of the preparation of the 1-adamantylethylmethylsulfonium ion.38340 It was claimed3* that no detectable acetolysis occurred after 138 h at 50 OC. Based on an extrapolated specific rate for I-AdSMe,', from the data of Table I, and applying a rate ratio of 4.2 (obtained from a comparison, at this temperature, of the rates of r-BuSMeEtf3* and t-BuSMez+8,41 acetolyses), it can be predicted that ca. 16% reaction should have been observed. A striking observation (Tables I and 111) is that the rates of solvolysis of the I-AdSMe2+ion are, at a given temperature, almost (39) Kevill, D. N.; Kamil, W . A. J . Org. Chem. 1982, 47, 3785. (40) Trost. B. M.; Hammen, R. F. J . Am. Chem. SOC.1973, 95. 962. (41) Swain, C. G.; Kaiser, L. E.; Knee, T. E. C. J . A m . Chem. SOC.1958, 8094092.
1582 J . Am. Chem. SOC.,Vol. 108, No. 7, 1986
Kevill and Anderson
Table IV. Percentage of Product Present“ as I-Adamantanol after the Solvolysis of I -Adamantyldimethylsulfonium Tri!luoromethanesulfonateb-c in Aqueous Ethanol and Selectivity Values ( S ) d
100.1 OC
70.6 OC %EtOHe
AdOH
100
3.6 19.9
96 90 88 86 84 82 80 70 60 40 20
42.8 47.3
S
S
AdOH
AdOH
7.8, 1.47 38.6
1.36
49.7 55.0 67.2 75.2 87.0 94.1
1.14 1.26 1.27 1.22 I .22 1.1 1
1.51 1.53
I. 6 16.3 34.5 40.8 45.1 49.5 51.5 52.3 64.0 73.3 85.3
S
AdOH
S
1.27 I .36 1.46 1.46 1S O 1.41 1.28 1.22 1.21 1.15
7.g 21.5 40.2 41.0 43.7 47.6 53.4 59.8 68.6 77.1 85.9
1.26 1.47 1.24 1.27 1.20 1.34 1.56 1.36 1.37 1.11
“Only I-adamantanol and I-adamantyl ethyl ether detected as products. &Concentration of 0.01 M. ‘A 10% excess of pyridine was present. dRatio of specific rate constants for reaction of intermediate ionic species with water and ethanol, respectively (see text). ‘On a volume-volume basis at 25.0 O C . /Corresponds to percentage of I-adamantanol impurity in the substrate. Table V. First-Order Rate Coefficients for the Solvolysis of the tert-Butyldimethylsulfonium Iona in 1,l , I ,3,3,3-Hexafluoro-2-propanol-Water Mixtures and a Comparison with Corresponding Values* for the I-Adamantyldimethylsulfonium Ion
106k, s-’ kcBuSMcl+/ Nd 49.7 ‘C 70.4 OC kl.AdSMe2te 0.823 18.4 2.60 (2.71)’ 97 -5.55 90 -3.78 1.28 26.2 3.76 70 -2.81 1.67 35.3 6.81 50 -2.48 1.82 43.1 8.85 “Trifluoromethanesulfonate salt; concentration of 0.0065 M and all runs performed in duplicate. bFrom Table 111. ‘By weight. dSolvent nucleophilicity based upon the specific rates of solvolysis of the Smethyldibenzothiophenium ion (ref 45). P A t 70.4 O C . (At 49.7 OC. %HFIPc
independent of the solvent composition. At 70.4 “C, for 41 different solvent compositions, the rates vary only by a factor of s-’ in tert-butyl alcohol to 7.09 X less than 7, from 1.09 X s-’ in 97% HFIP. Such a small variation in rate at a given temperature would be a remarkable coincidence unless the activation parameters were also virtually identical. Temperature effects were measured for seven solvents (Table I), and the values for both activation parameters were found to be, as expected, almost solvent-independent (Table 11). A large temperature dependence is reflected in the high enthalpies of activation, of ca. 35 kcal/mol. Reasonable rates are observed only because of a compensating entropy of activation of ca. +18 eu. The observation of almost constant specific rates of solvolysis is relevant to the that a “cavity term” should be included in the LFER for solvolyses of a substrate. This term relates to the energy changes associated with the change in the size of the cavity within the solvent, when the substrate is converted to the transition state. While the present observations do not rule out the influence of such a term upon the kinetics, they do strongly indicate, at least for 1-AdSMe2+solvolyses, that little variation in the magnitude of any cavity-term contribution accompanies a change in the solvent composition. A scale of solvent nucleophilicity, based upon the solvolysis of the triethyloxonium ion (a R-X+-type substrate), has been defined25by eq 6. This equation parallels eq 2 (with I = l ) , except that the scale of the solvent ionizing power (Y) is replaced by a 1% (k/kO)Et30+ = N
+ MY+
(6)
Y+ scale, governing the kinetic influence of the solvent at an initially positively charged leaving group, relative to the influence of the standard solvent (80% ethanol). More precisely, it can be (42) Kamlet, M . J.; Carr, P. W.; Taft, R. W.; Abraham, M . H. J . Am. Chem. SOC.1981, 103, 6062. (43) Abraham. M. H.; Kamlet, M. J.; Taft, R. W. J . Chem. SOC.,Perkin Trans. 2 1982, 923.
considered to govern, for a R-X+ substrate, all contributions toward changes in the reaction rate resulting from a change in the solvent other than those resulting from changes in solvent nucleophilicity. Initially the t-BuSMe2+ion was chosen as the standard for the Y+ scale.25 This paralleled the choice of tert-butyl chloride as the standard for the Yscale,” and it had the advantage that several values were a ~ a i l a b l e . ~Other ’ P values were estimated by using eq 7. However, a subsequent experimental extension to other
P = -0.09Y
(7)
solvents8 strongly indicated that the dominant factor controlling the variations in specific solvolysis rates of t-BuSMe2+ with changes in solvent composition was not the partial transfer of the positive change from sulfur to carbon but nucleophilic solvation of the incipient carbenium ion. Further, these observations indicated that t-BuSMeZf solvolysis was a poor choice for establishing a P scale for use within eq 6. Indeed, the m P corrections that had been made, fortunately rather small, were in the wrong direction. By analogy with the use of initially neutral adamantyl derivatives to establish Y scales,26an appropriate Y+ scale for use within eq 6 could be based upon 1-AdSMezf solvolysis, and P values are included within Tables I and 111. These values vary only from -0.319 to +0.495, and accepting that an m value of 0.55 is appropriateZ5for triethyloxonium ion solvolysis, the m P contribution varies only from -0.175 to +0.272. To a reasonable approximation, these contributions can be neglected compared and an N scale44 to the much larger variations in log (k/kO)Et30+, can be defined as in eq 8. Within Table V, four N values are included which are similarly calculated from the rates of solvolysis of the S-methyldibenzothiopheniumion.45
By comparison with literature data, it can be predicted that an sN2 attack a t a methyl group with expulsion of 1-adamantyl methyl sulfide will proceed much slower than sN1 attack with expulsion of dimethyl sulfide. At 100 O C , the specific hydrolysis s-l. From the rate of the trimethylsulfonium ion46is 12.8 X SKIfor data of Table I, an extrapolated value of 24.7 X hydrolysis of l-AdSMe2+,at 100 O C , can be calculated, a value greater than that for the trimethylsulfonium ion by a factor of 1.9 X lo5. The differing structures of the leaving sulfide molecules would be expected to lead to only modest rate differences. For solvolyses in aqueous ethanol, confirmation of the prediction of no significant attack at a methyl group came from the failure to (44) These NE,3o+values can be found within ref 25 and 39, where they are the first column of values within the Table I of both references. (45) A full range of N values, based upon this substrate, has been determined. These values will be reported elsewhere (Kevill, D. N.; Anderson, S. W., unpublished results). (46) Pocker, Y.; Parker, A. J. J . Org. Chem. 1966, 31, 1526.
Solvolysis of the 1-Adamantyldimethylsulfonium Ion
1
I +04
-04
J . A m . Chem. SOC.,Vol. 108, No. 7, 1986 1583
i .l-Ad$Me
A
t-BuiMe !-BuiMe
I
0 0
20 20
40 40 i
60 60
80 80
100 100
H,O(v/v)
Figure 1. Logarithmic plots of the specific rates of solvolysis of 1AdSMe,+OTf (at 70.4 "C) and f-BuSMe2*CI- (at 50.4 "C) as a function of the composition of an aqueous-ethanol solvent.
observe by GLPC any 1-adamantyl methyl sulfide within the
product^.^' Experiments carried out in the presence of a substantial concentration of tetraethylammonium chloride would, however, be expected to show a competing S N 2 attack by chloride ion. A second-order rate coefficient of 7.3 X L mol-] s-' has been reported4*for the bimolecular decomposition of trimethylsulfonium chloride in ethanol a t 100 O C , corresponding to an initial specific rate in the presence of a 0.16 M chloride ion of 1.17 X s-I. A specific rate of ethanolysis of l-AdSMe2+ a t 100.1 "C of, 16.4 X 1 0-5 SKIcan be obtained by an Arrhenius extrapolation of the data of Table I . Since the leaving groups differ, the specific rate of attack by chloride ion on the trimethylsulfonium ion can be considered as only a rough estimate of its specific rate of attack on I-AdSMe2+. Nonetheless the data do suggest that some attack is to be expected, especially a t lower temperatures. Solvolyses carried out in the presence of 0.16 M tetraethylammonium chloride did lead to the formation of I-adamantyl methyl sulfide, coupled with a fall off in the solvolytic specific rate with increasing extent of reaction. These are observations that would be expected to be a feature of S y 2 attack by chloride ion at a methyl group. Consistent with observations previously made for the ethanolysis ~ ~ 1-adamantyl chloride, of 1-adamantyl p - t o l ~ e n e s u l f o n a t e ,no which would have resulted from capture of the carbenium ion by chloride ion, was observed within the products. The S,2 attack of the halide ion a t a methyl group becomes the major pathway in a dipolar aprotic solvent, such as acetonitrile, and reaction of 1-AdSMe2+with tetra-n-butylammonium bromide in acetonitrile was used to prepare the I-adamantyl methyl sulfide needed for GLPC calibration. For seven binary solvent systems, the rates of solvolysis of both I-AdSMe2+ and t-BuSMe2+ have been determined over a wide range of solvent composition. The rates of solvolysis of the 1AdSMe2+ ion always vary in exactly the opposite manner to those reported, in Table V and e l s e ~ h e r e , for ~ ~the ~ ~rates , ~ ~ of solvolysis of the t-BuSMe2+ ion; several examples are given in Figures 1 and 2. Also, it can be seen that the rate variations accompanying changes in solvent composition are considerably lower for the I-AdSMe2+ ion. The marked differences in response to solvent variation of t BuSMe,' and l-AdSMe2+solvolysis rates are consistent with the claim8that the variations in t-BuSMe2+solvolysis rates are controlled primarily by solvent nucleophilicity considerations. In contrast, for 1-AdSMe2+ solvolysis, where specific rear-side nucleophilic solvation is not possible, one can consider the rate variations in terms of nonspecific solvation. The Hughes-Ingold approach49 can be used but with nucleophilic solvation being (47) Similarly, a careful study of the products after the acetolysis of r-BuSMeEt' indicated the absence of both r-BuSMe and r-BuSEt: Darwish. D.; Tourigny, G. J . Am. Chem. SOC.1972, 94, 2191. (48) Gleave, J. L.; Hughes, E. D.: Ingold, C. K. J. Chem.Soc. 1935, 236. (49) Reference 18; pp 345-350.
0
20
40
60
60
100
%TFE
Figure 2. Plots of the specific rates of solvolysis of 1-AdSMe,+OTf (at 70.4 "C) and t-BuSMe2+0Tf (at 50.0 "C) as a function of the composition of aqueous-TFE and TFE-EtOH solvents.
considered as the dominant effect rather than the ill-defined concept of polarity. Increased nucleophilic solvation will stabilize the more intensely charged initial state rather than the chargedispersed transition state, leading to a reduction in the solvolysis rate. However, as discussed previously,8 it has been suggested30 that S N 1 reactions in protic solvents have a very late transition state with 80-90% bond heterolysis. Under these conditions, R-X+ substrates will have appreciable transfer of charge and relatively little charge dispersion. Therefore, it is not surprising that as the solvent is varied, the rate changes are modest. The same modest effect must also operate in the solvolyses of t-BuSMe2+. However, for this substrate, it is swamped out by the stronger effect of stabilization of the incipient rert-butyl carbocation by specific nucleophilic solvation, an effect which has an opposite influence upon the reaction rate. In his extensive investigation of t-BuSMe2+ s o l v o l y ~ i s Hyne ,~~ foundsodpoor correlations of solvent-induced rate changes with Grunwald-Winstein Y values, Kosower 2 values, or the bulk dielectric constant. H e recognized that specific solute-solvent interactions, other than those based on polarity and solvating power, must play a critical role. In a discussion of solvolyses in N-methylformamide-water mixtures,s0d a nucleophilic role for N-methylformamide involving covalent bond formation to the a-carbonwas actually proposed.51 Surprisingly, Hyne and Jensen did not take the logical next step: to consider whether solvent nucleophilicity was also the general answer as to the nature of the proposed specific solute-solvent interactions. The observation of almost solvent-independent activation parameters for I-AdSMe2+ solvolyses is consistent with data for tert-butyldialkylsulfonium ion solvolyses. Almost constant activation parameters were observed for t-BuSEtMe' solvolyses in ethanol, acetic acid, and 50% acetic acid-acetic anhydride.47 The solvolyses of both t-BuSMe2+ and (a-pheny1ethyl)dimethylsulfonium ion in water and three aqueous-ethanol mixtures had activation parameters within narrow ranges: AH* from 32.4 to 33.0 kcal/mol and AS* from 15 to 18 eu.52 In direct comparisons, activation parameters for hydrolysis of l-AdSMe2+ of 34.2 kcal/mol and 16.2 eu (Table 11) can be compared with values for t-BuSMe,+ of 32.4 kcal/mol and 17.7 euS3or of 31.6 kcal/mol and 15.7 eu.54 Values of 36.2 kcal/mol and 20.9 eu for solvolysis (50) (a) Hyne, J. B. Can.J . Chem. 1961, 39, 1207. (b) Hyne, J. B.; Abrell, J . W. Can.J . Chem.1961, 39, 1657. (c) Hyne, J. B.; Jensen, J. H. Can. J . Chem. 1962.40, 1394. (d) Hyne, J. B.; Jensen, J. H. Can.J . Chem. 1963, 41, 1619. (51 J A similar nucleophilic role for formamides in tert-alkyl halide solvolyses had previously been proposed; see ref 2. (52) Hyne, J. B.; Golinkin, H. S. Can.J . Chem. 1963, 41, 3139. (53) Cooper,K. A.; Hughes, E. D.; Ingold, C. K.; Maw, G . A,; MacNulty, B. J. J . Chem. SOC.1948, 2049. (54) Leffek, K. T.; Robertson, R. E.; Sugamori, S. J . Am. Chem. SOC. 1965, 87, 2097.
1584 J. Am. Chem. SOC.,Vol. 108, No. 7, 1986 of l-AdSMe2+ in 80% ethanol can be compared with values for t-BuSMe,' of 32.1 kcal/mol and 18.2 eu.53 It appears that the specific nucleophilic solvation of the incipient carbenium ion, which increases the rates of t-BuSMe2+ solvolyses over those of corresponding 1-AdSMe2+solvolyses, is accompanied by a reduction in the activation energy and by little change in the entropy of activation. Possibly, the solvent molecules involved in rear-side solvation of the transition state are already suitably positioned at the initial state. In 95% aqueous acetone, the unchanged solvolysis kinetics in the presence of up to 0.1 M dimethyl sulfide (Table 111) indicates the absence of any appreciable external return. This is consistent with the lack of retardation upon addition of p-toluenesulfonate ~~ ion to the ethanolysis of 1-adamantyl p - t o l ~ e n e s u l f o n a t eand the lack of external capture of 1-adamantyl carbocations by chloride37or azide ionss5 in competition with capture by protic solvents. In highly nucleophilic solvents, the specific rate of solvolysis of the t-BuSMe2+ ions239341is considerably in excess of that of the l-AdSMe2+ ion. For example, a ratio of 210 is observed for ethanolysis a t 50.5 "C. As the nucleophilicity of the solvent is decreased, the rates of t-BuSMe2+ solvolysis decrease and those of l-AdSMe2+ solvolysis increase slightly. If these trends were to continue indefinitely, a situation would eventually be reached, in low nucleophilicity solvents, where the t-BuSMe,+ solvolysis was the slower. This situation can be avoided if the t-BuSMe2+ solvolysis abandons the specific nucleophilic assistance and solvolyzes by a mechanism essentially identical with that followed by 1-AdSMe2+ solvolyses. In a comparison of terf-butyl bromide and 1-adamantyl bromide s o l v ~ l y s e s it , ~ was found that as the solvent nucleophilicity decreased, the rates became very similar and, in 97% H F I P a t 25 O C , the rate ratio was only 2.56. It has, however, been sugg e ~ t e d ' ~that . ' ~ the variation in the rate ratio can alternatively be explained in terms of 1-adamantyl bromide being more sensitive to solvent electrophilicity than tert-butyl bromide; in this case, the low ratio in 97% H F I P is due not to its low nucleophilicity but to its high electrophilicity. The comparison of t-BuSMe,' and 1-AdSMe2+ specific solvolysis rates is of interest not only in its own right but also because it can help to decide between the two competing theories for the corresponding alkyl chloride solvolyses. The observation in 97% H F I P of low ratios for the specific solvolysis rates, 2.60 a t 70.4 O C and 2.71 a t 49.7 O C (Table V), gives strong support to the description for the halides based on solvent nucle~philicity.~ Solvent electrophilicity is not an important factor in the solvolyses of R-X+-type substrates, as illustrated by the essentially solvent-independent 1-AdSMe2+ specific solvolysis rates. The observation of a ratio identical with that for the alkyl bromide solvolyses is probably coincidental. Based on studies in solvents of low polarity (such as methylene halide56) or in the gas phase,57 it has been found that, in the absence of solvation, the 1-adamantyl carbocation is actually more stable than the tert-butyl carbocation. If a higher sensitivity toward nucleophilicity for t-BuX and not a higher sensitivity toward electrophilicity for 1-AdX is indeed the dominant factor determining variations in the rate ratio, one might expect that in low nucleophilicity solvents, it might be possible to reduce the t-BuX/l-AdX ratio to below unity. This has not been observed,58 and it might be an unrealistic goal in hydroxylic solvents since, even in the absence of specific nucleo( 5 5 ) Harris, J . M.; Raber, D. J.; Hall, R. E.; Schleyer, P. v. R. J . A m . Chem. Sor. 1970, 92, 5729. (56) Mirda, D.; Rapp, D.; Kramer, G. M . J . Org. Chem. 1979, 44, 2619. (57) (a) Staley, R. H.; Wieting, R. D.; Beauchamp, J. L. J . A m . Chem. SOC.1977, 99, 5964. (b) Houriet, R.; Schwarz, H. Angew. Chem., Inr. Ed. Engf. 1979, 18, 951. (c) Sharma, R. B.; Sen Sharma, D. K.: Hiraoka, K.; Kebarle, P. J . A m . Chem. SOC.1985, 107, 3747. (58) Since CH2C12was a solvent within which the 1-adamantyl carbocation was found to be the more stable, a comparison was made, at 70.3 "C, in 95% CH2CI2-5% TFE (by vol). However, the specific rates of solvolysis of 287 X lod s-' for t-BuSMe2+ and 4.26 X 10" s-' for l-AdSMe2*correspond to a ratio of 67.
Keuill and Anderson philic solvation, one would expect longer range dipolarity/polarizability effectsI5 to be more effective in stabilizing the incipient carbocation when they can operate at the rear side of a nonbridgehead system. In 96-60% aqueous ethanol, product studies (Table IV) show a constant selectivity, with average S values (eq 5) of 1.35 i 0.15 at 70.6 "C and 1.35 rt 0.11 at 100.1 OC. Below 60% ethanol content, a slight fall off in the S values was observed. The values are slightly lower than the values of 1.5 to 2.6 which have previously been observed in the same solvent mixtures for 1- and 2-adamantyl bromides and a r e n e s ~ l f o n a t e s , ~1-adamantyl ~.~~ picrate,33 2-adamantyl p e r ~ h l o r a t e and , ~ ~ 2-adamantyl trifluorom e t h a n e ~ u l f o n a t e . ~Values ~ have also been reported outside of this range: slightly lower for 1-adamantyl bromide35and higher for 2-adamantyl arene~ulfonates.~' The preference for reaction with water rather than with ethanol remains on going from the initially neutral leaving groups to the initially positively charged leaving group. The preference for reaction with the less nucleophilic water has been explained in terms of the relative abilities to hydrogen bond to the departed anion within a solvent-separated ion pair3' and in terms of a bulk effect, with the smaller water molecule better able to insert itself into the solvent-separated ion pair.59 More recent e x p e r i m e n t ~ ~ ~ , ~ ~ have not confirmed the reported3' variation of S with hydrogen bonding ability of the anion, and for aqueous ethanol a t least,3s the bulk theory would appear to present the simpler explanation. The bulk theory would also appear to offer the simplest explanation of the S value being greater than unity in the aqueous-ethanol solvolyses of I-AdSMe2+,with the need to consider the separation by the solvent of an ion and a molecule.
Experimental Section Materials. The purifications of acetone, fert-butyl alcohol, dioxane, ethanol, methanol, and 2-propanol were as previously described.,' The purifications of acetic acid,41 acetonitrile,60 formic acid,61 1 , I , 1,3,3,3hexafluor0-2-propanol,6~and 2,2,2-trifl~oroethanoI~~ were also prepared by use of previously reported procedures. The 1-adamantyl iodide was prepared from I-adamantanol and hydriodic acid as previously described.j7 Silver trifluoromethanesulfonate (Aldrich) and methyl sulfide (.4ldrich) were used without further purification. 1-AdamantyldimethylsulfoniumTriflate. Methyl sulfide (1.78 mL) was added to a solution of 6.06 g of I-adamantyl iodide in 100 mL of nitromethane. After 2 h, a solution of 5.94 g of silver triflate in 100 mL of nitromethane was added. An immediate precipitate of silver iodide occurred, and after stirring for 22 h, the mixture was filtered through a fine sintered glass funnel. The precipitate was washed with acetonitrile. Concentration of the total filtrate (rotary evaporator) gave a clear brown oil, which solidified to a tan paste. Repeated washing with ether led to a fine white powder (5.62 g, 70.3%): mp 112-1 18.5 0C;64IR (KBr disc) 3023,2925, 2913, 2860, 1255, 1171, 1149, 1031, 1004, 639 cm-I; P M R (CDCI,) 6 2.85 (s, CH,, 6 H ) 2.32 (s, 7 - H , 3 H), 2.07 (d, 0-H, 6 H , J = 2.3 Hz), 1.81 (d, 6-H, 6 H, J = 2.3 Hz). Anal. Calcd for C,,H,,F,O,S,: C , 45.07; H , 6.11; S, 18.51. Found: C, 45.01; H, 6.19; S, 18.37. Complete ethanolysis of a tared sample, in a sealed tube at 75 "C for 480 h, led to a titration against NaOMe in MeOH corresponding to 100.3% of the theoretical value. tert-Butyldimethylsulfonium Triflate. This salt was prepared as previously d e ~ c r i b e d : ) ~mp 128-137 ' C dec [lit.'9 mp 173-175 "C de^).^^ The P M R data were identical with those previously reported.39 Anal. Calcd for C7H15F30,S2:C, 31.34; H, 5.64; S, 23.90. Found: C, 31.34; H , 5.47; S, 23.74. 1-Adamantyl Methyl Sulfide. Tetra-n-butSlammonium bromide (1.33 g, Fluka) and 1 -adamantyldimethylsulfonium triflate (0.95 g) were reacted in refluxing acetonitrile for 24 h. Removal of solvent (rotary evaporator) gave a clear amber oil, which was partitioned between ether (25 mL) and water (25 mL). The aqueous layer was extracted with ether ( 3 X 25 mL) and the combined ether extracts were washed with water
(59) Ando, T.; Tsukamoto, S. Tetrahedron Lerf. 1977, 2775. (60) Kevill, D. N.; Dorsey, J . E. J . Org. Chem. 1969, 34, 1985. (61) Winstein, S.; Marshall, H. J . A m . Chem. SOC.1956, 74, 1126. (62) Bentley, T. W.; Bowen, C. T.; Parker, W.; Watt, C. I. F. J . Chem. Soc., Perkin Trans. 2 1980, 1244. (63) Rappoport, Z . ; Kaspi, J. J . A m . Chem. SOC.1974, 96, 4518. (64) It has been pointed out previously that for sulfonium salts, melting points are a poor test of purity. The usually observed decomposition points depend very much upon the rate of heating: Saunders, W. H.; Asperger, S . J . A m . Chem. SOC.1957, 7 9 , 1612.
J . Am. Chem. SOC. 1986, 108, 1585-1588 (3 X 25 mL) and dried over anhydrous MgS04. Removal of solvent (rotary evaporator) gave 0.34 g of crude product. Distillation under reduced pressure afforded 0.18 g (36%) of a clear, colorless liquid: bp 68-69 OC (0.55 mmHg); PMR (CDC13) 6 2.01 (s, CH3 + y-H, 6 H), 1.86 (d, 0-H, 6 H, J = 2.4 Hz), 1.70 (d, 6-H, 6 H, J = 2.4 Hz); IR (neat) 2910, 2860, 1655, 1450, 1345, 1300, 1045 cm-’. Anal. Calcd for Cl,H18S:C, 72.46; H, 9.95. Found: C, 72.36; H, 10.06. Kinetic Procedures. Each run was carried out by using nine sealed Kimble Neutraglas ampules. For runs with HFIP-containing solvents, each ampule contained 1 mL of solution, and for all other runs, each ampule contained 5 mL of solution. For runs in water, alcohols, and aqueous-organic mixtures, ampules were removed from the constant temperature bath at suitable time interals, quenched in an ice bath, and the contents rinsed with acetone into 25 mL of acetone, containing Lacmoid (resorcinol blue) indicator, cooled within a solid C0,-acetone slush bath. The acid produced was titrated against a standardized solution of sodium methoxide in methanol. The titration procedures for runs in acetic acid and formic acid and the calculation of the first-order
1585
solvolytic rate coefficients were as previously described.2s Product Studies. Ampules containing a ca. 0.01 M solution of 1AdSMe,+OTf in ethanol or the appropriate aqueous-ethanol solvent were allowed to react for at least ten half-lives at 70.6 or 100.1 OC. The products were directly analyzed by response-calibrated GLC, as previously described.34Only I-adamantanol and I-adamantyl ethyl ether were detected as products; in particular, no I-adamantyl methyl sulfide was detected. The 1-adamantyl methyl sulfide was, however, detected after reaction in ethanol in the presence of a large excess of anhydrous60 tetraethylammonium chloride.
Acknowledgment. This research was supported, in part, by the award of a Doctoral Research Fellowship to S.W.A. by the N I U Graduate School. D.N.K. thanks T. W . Bentley (University College of Swansea, University of Wales) and H. M. R. Hoffmann (Universitat Hannover) for hospitality while this manuscript was being prepared.
Geometries and Energies of the Fluoroethylenes David A. Dixon,* Tadamichi Fukunaga, and Bruce E. Smart Contribution No. 3754, E . I. d u Pont de Nemours & Co., Central Research & Development Department, Experimental Station, Wilmington, Delaware 19898. Received May 3, 1985. Revised Manuscript Received October 2, I985
Abstract: The geometries of the fluorinated ethylenes have been gradient optimized at the S C F level with a double-< plus polarization function on carbon (DZ+Dc) basis set. The C==C and C-F bond lengths for C2H4,C,H,F, CH,CF2, cis-CHFCHF, trans-CHFCHF, and C2F4have been optimized at the configuration-interaction level, including all single and double excitations (CI-SD). Good agreement with the original electron diffraction work was found. The values for r(C=C) and r(C-F) are found to decrease with increasing fluorine substitution. Correction factors to the S C F values are discussed and an estimated structure for C2HF3is given. Isodesmic reaction ethalpies for the fluoroethylenes have been calculated with (DZ+D,-) and double-< plus polarization function (DZ+P) basis sets. Both basis sets gave comparable results in satisfactory agreement with experiment. The calculated heats of formation of cis- and trans-CHFCHF are -71.0 and -70.0 kcal/mol, respectively, which compare to the experimental values of -70.8 f 3.1 and -69.4 f 3.1 kcal/mol. Ionization potentials and dipole moments of the fluoroethylenes also have been calculated and are compared to experimental data.
Fluoroethylenes are very simple compounds and are extremely important monomers, but their structures are not unequivocally established.’ In a systematic electron diffraction study, Bauer and co-workers2 found that the C=C bond length decreases as fluorines are substituted for hydrogen in ethylene. The C==C bond length of 1.315 A in CH2CF2from a microwave study3a agrees with an electron diffraction value of 1.316 A.* The bond distances in C2H,F as determined by microwave3b and electron diffraction were also in good agreement. Recently, however, the structures of the fluoroethylenes have been redetermined by using a combination of electron diffraction and microwave and the results did not show the expected general trend for a decrease in (1) Smart, B. E. In ‘Molecular Structures and Energetics”;Liebman, J. F., Greenberg,A., Eds.; Verlag Chemie: Deerfield, FL, 1986; Vol. 3, Chapter 4.
(2) Carlos, J. L.; Karl, R. R.; Bauer, S. H. J . Chem. Sor., Faraday Trans 2, 1974, 70, 177. (3) (a) Laurie, V. W.; Pence, D. T. J . Chem. Phys. 1963, 38, 2693. (b) Laurie, V. W. J . Chem. Phys. 1961, 34, 291. (4) Huisman, P. A. G.; Mijlhoff, F. C.; Renes, G. H. J . Mol. Struct. 1979, 5 1 . 191. 1~
(5) Spelbos, A.; Huisman, P. A. G.; Mijlhoff, F. C.; Renes, G. H. J . Mol. Struct. 1978, 44, 159. (6) VanSchaick, E. J. M.; Mijlhoff, F. C.; Renes, G. H.; Geise, H. J. J . Mol Struct. 1974, 21, 17. (7) Mijlhoff, F. C.; Renes, G. H.; Kohata, K.; Oyanagi, K.; Kuchitsu, K. J . Mol. Struct. 1977, 39, 241. ( 8 ) Mom, V.; Huisman. P. A. G.; Miilhoff, F. C.; Renes. G. H. J . Mol. Struct. 1980, 62, 9 5 .
0002-7863/86/ 1508-1 585$01.50/0
the value of r(C=C) with increasing substitution of fluorine. In fact, the C=C bond lengths in CH2CH2,CH2CF2,and CHFCF, were reported to be identical within experimental error.’,* The similarity in values for r(C=C) and r(C-F) further complicated the analysis of the electron diffraction data. As part of our general theoretical study of fluorocarbons, we have optimized the structures of the fluoroethylenes a t the SCF level using a double-{ (DZ) basis set augmented by polarization functions on carbon (DZ+D,-). T o obtain accurate geometries and resolve the discrepancies among the experimental measurements, we subsequently optimized the C=C and C-F bonds for C2H4, C2H3F,the difluoroethylenes, and C2F4with correlated wave functions starting from the optimum SCF structures. The results are compared with those from previous a b initio calculations. To further test the reliability of the DZ+Dc basis set, the total energies of the fluoroethylenes were computed and used to calculate isodesmic reaction enthalpies, which are known experimentally. Ionization potentials and dipole moments also were calculated and compared to experiment. Calculations. The calculations were performed with the H O N D O program’ package on D E C VAX/ 11-780 and IBM 3083 computers. Geometries were optimized a t the SCF level with the use of gradient techniques.I0 The correlated wave (9) (a) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976,65, 11 1. (b) King, H. F.; Dupuis, M.; Rys, J. National Resource for Computer Chemistry Software Catalog; Program Q H 0 2 (HONDO), 1980; Vol. 1.
0 1986 American Chemical Society
